What Slim said.
I would also add that the PG apparently inhibits mold growth on your credo sponge. With 50/50, you can sleep easy, it does the work for you. If your hygrometers are off, don't worry, just use them as a reference. When they start to head south, no matter what they are reading, add distilled water. It's better to add distilled water before the credo dries, however.